chore: remove docs/superpowers planning artifacts

[brianschroeder]

Summary

• Removes docs/superpowers/specs and docs/superpowers/plans — these were brainstorming/planning artifacts from building this repo, not something needed for its ongoing life as a plugin

Note

• PR feat: add append-time dedup/supersede check #2 (still open) adds another spec file under docs/superpowers/specs/ — once both merge, that file will need removing too in a follow-up

Test plan

• validate CI check passes (no skill/manifest files touched)

[brianschroeder]

Folding this into #2 instead.